Magnetic resonance imaging (MRI) provides exquisite soft tissue contrast. Positron emission tomography provides exquisite molecular and biochemical information, but often lacks anatomic contrast. This latter fact has led to the development of combined PET-CT (computed tomography) scanners in order to use x-ray CT to provide the anatomic contrast lacking in the PET. The possibility of combining an MRI and PET scanner has been experimentally documented, however the high magnetic field creates many challenges in engineering the final instrument. This proposal seeks to obtain funding to purchase a prototype PET-MRI scanner to be built by Siemens-CPS Instrumentation. Although a first generation instrument, this instrument would allow for collection of high quality soft-tissue anatomic contrast using a 1.5T scanner while simultaneously acquiring high quality PET data with a spatial resolution on the order of 3 mm from a concentric PET detector. This proposal represents a collection of NIH-funded projects, all of which would greatly benefit from the added anatomic and metabolic information that the MRI can provide to state-of-the-art PET studies in a number of different disease processes. This scanner would afford us the opportunity of working out in detail the limitations and advantages of such a system to help tailor future generation scanners. This proposal reflects the belief of the allied investigators that the future of PET scanning may well be found in such devices, and that immediate testing, in both basic science and clinical settings, will afford us an unparalleled opportunity to improve the quantitative accuracy of PET scanning. [unreadable] [unreadable] [unreadable]